1. Field of the Invention
The present invention relates to a contact charger and image forming apparatus provided with same, such as electrophotographic copying machines, printers, facsimile and the like.
2. Description of the Related Art
In image forming apparatus such as electrophotographic copying machines, printers, facsimiles and the like, the surface of an electrostatic latent image carrying member such as a photosensitive drum or the like is charged by means of a charging device. The charged region of said surface is subjected to image light exposure to form an electrostatic latent image thereon, said latent image is subsequently developed so as to be rendered visible, transferred onto a recording medium, and fixed on said recording medium.
Various types of such charging devices are known. Such chargers can be broadly divided into corona chargers which utilize a corona discharge via a corotron system, scorotron system, serrated electrode array system or the like, and contact chargers wherein a charging member such as a roller, brush, film, belt or the like is brought into contact with the surface of the electrostatic latent image carrying member.
Chargers that utilize a corona discharge are advantageous insofar as they provide stabilized charging, however they also have certain disadvantages in that they produce large amounts of ozone, which leads to deterioration of the electrostatic latent image carrying member, and adversely affects humans. Thus, attention has become focused on contact chargers which produce markedly less ozone compared to corona chargers.
Among the aforesaid charger types, contact charger having a roller configuration have been made practical via a function-separation construction having a conductive substrate, a conductive elastic layer formed on the conductive substrate and a conductive resin layer formed on the conductive elastic layer.
The conductive substrate provides a voltage supplying function to impart a high voltage to the entire roller, and a pressure imparting function for maintaining the contact between the roller and the electrostatic latent image carrying. member. The conductive elastic layer provides a current maintaining function to prevent voltage drops from the interior region of the roller to the exterior region thereof by a suitable electrical resistance value setting, and a cushion function for maintaining contact between the roller and latent image carrying member with a wide nip therebetween. The conductive resin layer provides a breakdown prevention function to prevent abnormal discharge of the roller in the defective areas of the latent image carrying member produced during manufacture via a suitable electrical resistance value setting, a release function to prevent adhesion of developer and dust, and a friction resistance function to prevent damage incurred via the rubbing with the latent image carrying member.
Contact chargers of the aforesaid roller type have been made practical in the area of low processing speed (peripheral speed of latent image carrying member). However, the contact charger of the roller type has not been made practical in the high speed area which is strongly desired for low ozone production, e.g., the area of 35 pages/minute and faster, in other words, in the area of the processing speed over 22 cm/second.
Contact chargers of the roller type having a conventional laminar construction are difficult to adapt to the high speed area due to the difficulty of assuring stable contact with the electrostatic latent image carrying member. That is, by randomly providing a conductive resin layer superimposed over a conductive elastic layer, the rotational speed of the roller is increased, and the conductive elastic layer which has a high degree of cushioning becomes deformed through contact with the latent image carrying member, such that the expansion of the conductive elastic layer protecting the surface of the roller cannot occur, and, consequently, a minute bounce begins to be generated by the roller which produces parallel striation-like image noise in the lengthwise direction of the roller. Simply increasing the pressure of the roller abutting the latent image carrying member to eliminate the problem of the aforesaid minute bounce of the roller readily produces image noise in the high-pressure contact area of the latent image carrying member, and leads to a further disadvantage inasmuch as the strength of the photosensitive member must be increased in relation to the high pressure used, thereby increasing cost of the photosensitive member.